Condenser with first and second photodetectors with three sections each and having focal points before and after the surface of detectors

ABSTRACT

To provide a light receiving device, a light detecting device, and an optical signal reproducing device each of which allows one to perform many different computations in detecting aberration amounts and focus error quantities without requiring exact position relations between laser light to be received and light receiving elements, and between the light receiving elements. First and second light receiving elements  43   +  and  43   −  are used which receive condensed light at positions equidistantly spaced from an focal point X before and after the light images, respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an aberration amount detecting device,and a light receiving device. More particularly, the invention relatesto a device for detecting light at positions before and after a focalpoint of condensed light.

2. Description of the Related Art

Nowadays, the number of types of optical recording media is increasing.The protecting layers having some different thicknesses are alsoprovided. In reading out information from an optical disc, when athickness of the protecting layer is deviated from a design value, or itis inclined to an optical axis of the object lens, a sphericalaberration occurs in the wave front of an impinging light beam, and ithinders the image formation of a micro-spot.

Recently, some short-wave, high recording density optical heads are eachdesigned to have a high numerical aperture of 0.85 or higher in order toreduce a diameter of a beam spot to read information. In this case, anaberration occurring in the optical system, in particular a sphericalaberration caused by a thickness error of a protecting layer of therecoding medium greatly affects the image forming spot diameter. Thisaberration must be detected and corrected by some methods.

For this background, many aberration detecting methods have beenproposed. JP-A-1998-214436 and JP-A-2000-57616 disclose the followingtechnique. Light receiving elements are provided at a focal point. Thelight receiving elements are located at the inner and outer positionswith respect to an optical axis of the beam. The light receiving areasof the inner and outer positions are each quartered by crossed divisionlines. By computing the output signals from those divided areas, a focalerror signal and a spherical aberration signal are obtained.JP-A-2000-171346, JP-A-2002-39915, and JP-A-2002-55024 disclose anothertype of technique. Light receiving elements are provided at a pointwhere light images. An incoming light beam is split into an inner lightbeam and an outer light beam. Those light beams are led to the lightreceiving elements, respectively. By computing the output signals fromthose divided areas, a focal error signal and a spherical aberrationsignal are obtained.

FIG. 1 is a diagram showing a light receiving device in use for theconventional aberration amount detecting mentioned above. A light source103 generates laser light. Laser light emitted from the light source 103travels through a predetermined path and reaches an optical disc (notshown). The lights reflected on the reflecting surface of the opticaldisc passes through a collimator lens 104. The reflected lights areincident on the areas of a hologram 101 and diffracted every reflectedlight, and image at predetermined positions on a light receiving element102.

The reflected light that is incident on an inner peripheral area 101 aof the hologram 101, images on a division line (not shown) provided onan inner peripheral light-amount detecting area 102 a of the lightreceiving element 102. The reflected light incident on the outerperipheral area 101 b of the hologram 101 images a division line (notshown) provided on an outer peripheral light-amount detecting area 102 bof the light receiving element 102. The reflected lights that areincident on tracking error signal areas 101 c and 101 d image ontracking error signal detecting areas 102 c and 102 d, respectively.Paths of the reflected lights from the tracking error signal areas 101 cand 101 d to the tracking error signal detecting areas 102 c and 102 d,are not illustrated.

Already described, the inner and outer peripheral light-amount detectingarea 102 a and 102 b are each divided into two sub areas by a divisionline. An amount of spherical aberration of the reflected light iscomputed according to the amounts of light from the two sub-areas and byusing all the electric signals derived from the sub areas.

The aberration amount detecting device using the conventional lightreceiving device uses light in the vicinity of the image formingposition to detect the aberration amount. For this reason, in a statethat the aberration amount is almost zero, the reflected light mustalmost image at each light receiving area. The light receiving areasprovided at least for detecting the aberration amount are each divided.It is necessary to position a microspot of which the diameter rangesfrom several μm to over ten μm on the division line at each lightreceiving area. Where each light receiving area is quartered by crosseddivision lines, it is necessary to position the center of the microspotat a nodal point of the crossed division lines.

Accordingly, it is difficult to set a positional relation between thehologram 101 and the light receiving areas of the light receivingelement 102. In a case where the light receiving areas of the lightreceiving element 102 are integrally formed as shown, it is necessary toexactly set the orientations of the light receiving element as well asthe position relation. This is very difficult to realize such by themanufacturing.

Since the spot diameter is extremely small, it is difficult to split thelight containing the center of light from the peripheral light notcontaining the same, and apply those split lights to different lightreceiving areas. Even if the light is split into two different lights,and those split lights are successively received by the two lightreceiving areas, the number of divisions is at most two. Accordingly,many restrictions are imparted to the formulae for computing theaberration amount and the focus correction amount.

When device temperature varies, the frequency of the laser lightslightly shifts from its correct value, a direction in which the laserlight is diffracted by the hologram slightly changes, and hence thelaser light lands at a position slightly different from the divisionline.

Also when the laser light used for the optical pickup used in theaberration amount detecting device is slightly shifted in the radialdirection of the optical disc by the tracking servo, the laser lightalso lands at a position out of the division line.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a lightreceiving device, a light detecting device, and an optical signalreproducing device which are easy to manufacture, and operable free froma shift of an focal point of light caused by ambient temperaturevariation.

A light receiving device having a first light receiving element forreceiving condensed light before the condensed light images, and asecond light receiving element for receiving the condensed light afterthe condensed light images. The first and second light receivingelements are disposed at positions equidistantly spaced from an focalpoint of the condensed light and generate electrical signals based onlight received by the light receiving elements. In the light receivingdevice, each of the fist and second light receiving elements includes: afirst light receiving area for receiving light of a central part of thecondensed light; a second light receiving area adjoining a side of thefirst light receiving area and for receiving light not containing thelight of the central part of the condensed light; and a third lightreceiving area adjoining a side of the second light receiving area whichis opposite to the first light receiving area.

Thus, the unique feature that the light receiving elements are notlocated at positions near the focal point of the received light, lessensthe requirements for the positional precision of the light receivingelements. With the first light receiving area for receiving light of acentral part of the condensed light, the second light receiving area forreceiving light not containing the light of the central part of thecondensed light, and the third light receiving area for receiving lightof a fringe part of the condensed light, the weak light of a fringe partof the image can be effectively utilized. Accordingly, in detecting theaberration amount and the focus correction amount, many differentcomputations can be performed according to conditions by utilizing theoutput signals output from the light receiving areas of both the lightreceiving elements. In the specification, the term “adjoin” means thatthe adjacent light receiving areas are disposed in contact with eachother, and also that those light receiving areas are located adjacent toeach other while being spaced from each other by a distance necessaryfor separating the light receiving areas one from the other.

In a preferred embodiment, a light receiving device according to claim1, wherein the first light receiving element and the second lightreceiving element are symmetrical with respect to a point locatedbetween the first and second light receiving elements. Therefore, imagesof the condensed light before and after the light images are pointsymmetrical to each other. Therefore, the light receiving elementslocated on both sides of the focal point are able to receive light underalmost the same conditions.

In the light receiving device, the width of the first light receivingarea is preferably larger than that of the second light receiving area.By so selected, there is no chance that the optical axis shifts to anend of the first light receiving area, not a boundary between the firstand second light receiving areas. This increases margins for error ofmounting positions of the light receiving elements and the shift of theoptical axis caused by ambient temperature of the device including thelight receiving device.

In the light receiving device, the width of the third light receivingarea is preferably larger than the width of each of the first and secondlight receiving areas. By so selected, the third light receiving area isable to receive even the weak light part around the fringe of the image,and hence, capable of producing an output signal of high level.

In the light receiving device, the total width of the widths of thefirst and second light receiving areas is preferably 20 to 50 μm.Further, it is preferable the widths of the first and second lightreceiving areas are each 10 to 30 μm. The width of the third lightreceiving area in a direction vertical to a boundary line between thesecond and third light receiving areas is preferably 40 to 180 μm.

In the light receiving device, the first and second light receivingelements are located at positions spaced apart from the focal point ofthe light by a distance of 0.1 to 0.5 mm. With such an arrangement,images are obtained at the first to third light receiving areas, and asufficiently large output signal is produced.

In the light receiving device, the first light receiving elementreceives one of lights spectrally split by a splitting means, and thesecond light receiving element receives the other split light. By sospectrally splitting light, it is easy to receive the lights before andafter the light images. The splitting means may be any of a half prism,a parallel plane element, and a hologram.

In the light receiving device, when the splitting means is a hologram, aboundary line between the first and second light receiving areas and aboundary line between the second and third light receiving areas aresubstantially vertical to the parallel lines of a grating of thehologram. When a wavelength of laser light passing through the hologramdeviates from its correct value, a diffraction angle of the laser lightchanges in a direction vertical to the parallel lines of the grating ofthe hologram. In such an arrangement, the respective light receivingareas receive fixed amounts of light also when the wavelength of thereceived laser light changes owing to ambient temperature of the relateddevice. Sometimes one or some of the parallel lines of the grating areslightly bent. In the specification, “substantially vertical to theparallel lines of the grating” means that the boundary lines aresubstantially vertical to an average of the directions of the lines ofthe grating containing the slightly bent parallel grating line.

Where each light receiving element receives light the light reflectedfrom the optical recording medium when the optical recording medium isirradiated with light, the light receiving device is suitable in usefor, for example, a pickup device of the optical disc. In the lightreceiving device, a boundary line between the first and second lightreceiving areas and a boundary line between the second and third lightreceiving areas are substantially vertical to a direction of a componentof the reflected light in a track direction of the optical recordingmedium. This arrangement suppresses adverse effects which will beproduced when the focus lens 39 shifts in a direction (radial direction)vertical to the track of the optical disc 2 by the follow-up action ofthe tracking-servo. Specifically, if the track becomes eccentric and theobjective lens shifts, the image of the reflected light sometimes shiftsin a direction vertical to the track direction. Even in such case,substantially fixed images are received at the light receiving areas andstable output signals are secured.

In another embodiment, the light receiving device further comprisesaberration correction means for correcting a quantity of aberration ofthe light reflected from the optical recording medium when the opticalrecording medium is irradiated with light in accordance with anaberration correction drive current based on the output signals of thefirst and second light receiving elements.

According to another aspect of the invention, there is provided a lightdetecting device includes an aberration amount detecting circuit fordetecting an aberration amount by using the output signals of the firstand second light receiving elements of any of the light receivingdevices described above. When the reflected light contains a sphericalaberration caused by manufacturing error of the optical disc, the lightdetecting device having such an aberration amount detecting circuit iscapable of acquiring its information satisfactorily.

In the light detecting device, the aberration amount is detected byusing any of the following equations:AB=a ₊ −a ⁻AB=(a ₊ +b ⁻)−(b ₊ +a ⁻),AB=(a ₊ +b ⁻ +c ⁻)−(a ⁻ +b ₊ +c ₊),AB=(a ₊ +b ₊ +c ⁻)−(a ⁻ +b ⁻ +c ₊),AB=(a ₊ +b ⁻ +c ₊)−(a ⁻ +b ₊ +c ⁻),AB=(a ₊ +b ₊)−(a ⁻ +b ⁻)where a₊, b₊, and c₊ are output signals derived from the first to thirdlight receiving areas of the first light receiving element, a⁻, b⁻, andc⁻ are output signals derived from the first to third light receivingareas of the second light receiving element. By using such equations, ifAB=a₊−a⁻, AB=(a₊+b⁻)−(b₊+a⁻), or AB=(a₊+b⁻+c⁻)−(a⁻+b₊+c₊), a sensitivityof sensing the aberration amount is high. If AB=(a₊+b₊)−(a⁻+b⁻⁻) orAB=(a₊+b⁻+c⁻)−(a⁻+b₊+c₊), a margin for the optical axis shift is large.If AB=(a₊+b₊+c⁻)−(a⁻+b₊+c⁻), the sensitivity of sensing the aberrationamount is further increased.

The light detecting device may include a focus correction amountdetecting circuit for detecting a focus correction amount by using theoutput signals of the first and second light receiving elements of anyof the light receiving devices described above. The light detectingdevice thus constructed is capable of properly detecting a focuscorrection amount.

In the light detecting device, the focus correction amount FO isdetected by using any of the following equations:FO=a ₊ +a ⁻FO=(a ₊ +b−)−(b ₊ +a ⁻),FO=(a ₊ +b ⁻ +c ⁻)−(a ⁻ +b ₊ +c ₊),FO=(a ₊ +b ₊ +c ⁻)−(a ⁻ +b ⁻ +c ₊),FO=(a ₊ +b ⁻ +c ₊)−(a ⁻ +b ₊ +c ⁻),FO=(a ₊ +b ₊)−(a ⁻ +b ⁻)wherea₊, b₊, c₊ are output signals derived from the first to third lightreceiving areas of the first light receiving element, a⁻, b⁻, and c⁻ areoutput signals derived from the first to third light receiving areas ofthe second light receiving element. Thus, the focus correction amountand the aberration correction amount are computed independently.Therefore, there is no interference between the correction signals. Anyparticular limitation is not imparted to choice of the detectionformulae. Those formulae may be appropriately chosen while consideringits balance with the aberration correction signal.

An optical signal reproducing device constructed according to theinvention receives light the light reflected from an optical recordingmedium when the optical recording medium is irradiated with light, andreproduces a signal from the optical recording medium, by using theoptical signal reproducing device includes the light detecting devicedescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a light receiving device in use for aconventional aberration amount detecting device.

FIG. 2 is a block diagram showing an optical disc reproducing devicewhich is one form of an optical signal reproducing device constructedaccording to the invention.

FIG. 3 is a model diagram showing an optical system of an optical pickupas one form of an optical signal detecting device constructed accordingto the invention.

FIG. 4 is a diagram showing a specific example of a light receivingblock shown in FIG. 3.

FIG. 5 is a diagram showing another specific example of the lightreceiving block D shown in FIG. 3.

FIG. 6 is a diagram showing yet another specific example of the lightreceiving block shown in FIG. 3.

FIG. 7 is a diagram showing a light receiving area of a first or secondphoto diode.

FIG. 8 is a diagram showing in detail the light receiving block shown inFIG. 6.

FIG. 9 is a diagram showing in model form preconditions for thecomputation of an aberration amount and the like by using a hologramconsisting of three segmental areas, which is described referring toFIG. 8.

FIG. 10 is a diagram showing another optical pickup as one form of anoptical signal detecting device constructed according to the invention.

FIG. 11 is a diagram showing still another optical pickup as one form ofan optical signal detecting device constructed according to theinvention.

FIG. 12 is a model diagram showing in detail a semiconductor laserdevice capable of emitting and receiving light shown in FIG. 11.

FIG. 13 is a diagram showing images appearing on a light receivingelement, and distributions of light intensity along a lateral axispassing through the center of the image when a protecting layer of theoptical disc is thinner than a

FIG. 14 is a diagram showing images appearing on a light receivingelement, and distributions of light intensity along a lateral axispassing through the center of the image when a protecting layer of theoptical disc has a predetermined thickness of 0.1 mm.

FIG. 15 is a diagram showing images appearing on a light receivingelement, and distributions of light intensity along a lateral axispassing through the center of the image when a protecting layer of theoptical disc is thinner than a predetermined thickness of 0.6 mm by 20μm.

FIG. 16 is a graph showing variations of signals with respect tothickness errors of a protecting film which are computed by use of aplurality of computing formulae when a distance between an focal pointand a light receiving element is 0.162 mm.

FIG. 17 is a graph showing variations of signals with respect tothickness errors of a protecting film which are computed by use of aplurality of computing formulae when a distance between an focal pointand a light receiving element is 0.243 mm.

FIG. 18 is a graph showing variations of focus signals with respect to adistance between the focal point and the light receiving element, withparameters each being the total width of the first and second lightreceiving areas.

FIG. 19 is a graph showing variations of focus signals with respect tothe total width of the first and second light receiving areas, withparameters each being distance between the focal point and the lightreceiving element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be describedwith reference to the accompanying drawings. Diagrams to be used forexplanation are depicted in model form by modifying the scales forsimplicity purposes. It should be understood that such depiction ofdiagrams does not affect the technical idea of the invention in any way.The present invention will be described by using an optical signalreproducing device in which blue laser light of 405 nm in wavelength isused and a numerical aperture for the light receiving element side is0.1.

FIG. 2 is a block diagram showing an optical disc reproducing device 1which is one form of an optical signal reproducing device. The opticaldisc reproducing device 1 functions to reproduce information recorded onan optical disc 2, which is directly or indirectly chucked on a spindlemotor 3 by means of a chucking means (not shown)

The optical pickup 30 is a light receiving device for reading outinformation from the optical disc and outputting an electrical signalindicative of the readout information. The optical pickup is provided ona chassis 28, and may be moved in a radial direction of the optical disc2 by a slide motor 27.

An electrical signal output from the optical pickup 30 is input to an RFamplifier 4 which produces an RF signal as data reproducing signal, afocus error signal, a tracking error signal, and an aberration amountsignal. In the RF amplifier 4, electrical signals from the opticalpickup 30 are input to the calculator 5. The calculator 5 adds togetherall electrical signals from the photo diodes as light receiving elements(to be described later) to thereby produce an RF signal. The RF signalis input to a waveform equalizer 6 which in turn waveform equalizes theRF signal to suppress the waveform interference and the like. A signaloutput from the waveform equalizer 6 is input to a wave shaper 7 whereit is converted into a pulse signal. The pulse signal is input to asignal processing circuit 11. The signal processing circuit 11 executesthe processings of clock reproduction, sync detection, datademodulation, error detection, error correction and others. A signaloutput from the signal processing circuit 11 is applied to a D/Aconverter 12. In turn, the D/A converter 12 converts the received signalinto an analog signal, and outputs the analog signal through an outputterminal 13.

In the RF amplifier 4, electric signals output from the photo diodes aslight receiving elements, in addition to data output signals, are inputto a focus error detecting circuit 8, a tracking error detecting circuit9, and an aberration detecting circuit 10. Those circuit blocks executecomputing processings to produce a focus error signal, a tracking errorsignal, and an aberration signal, respectively, and output those signalsto a servo processing circuit 16.

The servo processing circuit 16 includes a focus control circuit 17, atracking control circuit 18, a aberration control circuit 19, and aslide control circuit 20. The servo processing circuit 16 receives thefocus error signal, the tracking error signal, the aberration amountsignal and the like from the RF amplifier 4, and generates signals foradjusting the focusing, tracking, and aberration of the optical pickup30 and for slide adjusting a position of the optical pickup 30, andsends those servo signals to a focus driver 22, a tracking driver 23, anaberration correction driver 24, and a slide driver 25. The servoprocessing circuit 16 further includes a spindle control circuit 21which sends a spindle servo signal to a spindle driver 26.

Upon receipt of the servo signal from the servo processing circuit 16,the tracking driver 23 generates a tracking drive current to drivetracking means in the optical pickup 30 and a tracking correctionoperation is performed, as will be described later. Upon receipt of theservo signal from the servo processing circuit 16, the focus driver 22generates a focus drive current for moving a focus lens of the opticalpickup 30 in focusing direction. Upon receipt of the servo signal fromthe servo processing circuit 16, the aberration correction driver 24generates an aberration correction drive current for driving aberrationcorrecting means (to be described later) in the optical pickup 30 and anaberration correction operation is performed based on the currentsignal.

Further, the slide driver 25 generates a current for sliding the opticalpickup 30 by the slide motor 27 in accordance with a slide servo signal.The spindle driver 26 generates a current for controlling rotation of aspindle motor 3 in accordance with a spindle servo signal.

A system controller 14 receives signals from an external switch 15 andthe signal processing circuit 11, and sends a control signal to theservo processing circuit 16 and others.

The optical signal reproducing device has been described by use of theoptical signal reproducing device. Further, the optical signalreproducing device may be realized in the form of arecording/reproducing device which is capable of recording opticalsignals. In this case, a predetermined circuit must be additionallyused. It will be readily understood that the optical signal reproducingdevice is realized in any of other suitable forms.

FIG. 3 is a model diagram showing an optical system of an optical pickupas one form of an optical signal detecting device constructed accordingto the invention. The optical pickup to be discussed below is operablefor both information reproducing and recording purposes.

The semiconductor laser device 31 as a light source generates blue laserlight whose wavelength is 405 nm. The laser light emitted from thesemiconductor laser device 31 is diverged to an appropriate extent, andthen it is collimated by a collimator lens 32. The laser light isgenerated in a state that it is elliptical in cross section or as viewedin a plane perpendicular to the light traveling direction. To reshapethe laser light, the laser light is caused to enter a beam forming prism33. The beam forming prism 33 reshapes the laser light by itsrefraction, so that the laser light takes a substantially complete roundin cross section. Immediately after emitted from the semiconductor laserdevice 31, the laser light is inclined at a predetermined angle to theoptical disc 2. After passing through the beam forming prism 33, thelaser light is refracted to a direction perpendicular to the opticaldisc 2.

To reproduce an information signal from the optical disc 2, thesemiconductor laser device 31 emits laser light of a fixed output power.To record information into the optical disc 2, an intensity of the laserlight emitted varies in accordance with a signal to be recorded. In theinvention, a wavelength of the laser light emitted from thesemiconductor laser device 31 differs in accordance with the differentstandards employed, and is not limited to a specific wavelength.

After passing through the beam forming prism 33, the laser light entersa polarized light beam splitter 34 as a sort of half prism. About 10% ofthe laser light emitted from the semiconductor laser device is reflectedtoward a power-monitor photo diode 40 by the polarized light beamsplitter 34, while the remaining laser light, i.e., about 90% of theemitted laser light, is used for reading out information from theoptical disc. The power-monitor photo diode 40 monitors an intensity ofthe laser light emitted from the semiconductor laser device 31, andfeeds back the result of the monitoring to the semiconductor laserdevice 31 through a circuit (not shown) The monitoring function by thepower-monitor photo diode 40 is not essential to the invention, but willwork well in particular, when the optical pickup 30 is operated forrecording purposes.

The laser light emitted from the polarized light beam splitter 34 passesthrough the ¼ wavelength plate 35. When passing through the ¼ wavelengthplate 35, the laser light is circularly polarized, and the circularlypolarized light flux is incident on the optical disc.

The laser light having passed through the ¼ wavelength plate enters theaberration correction lens 37. In some cases, the aberration correctionlens consists of a single lens, and in other cases, the aberrationcorrection lens consists of combination of plural lenses. Either ofthose types of aberration correction lenses may be used in theinvention. The aberration correction lens 37 is held with an aberrationcorrection actuator 36 containing, for example, a combination of a coiland a magnet as a part, whereby those components form aberrationcorrection means. With the aberration correction means thus constructed,the aberration correction lens 37 is adjusted in an aberrationcorrection direction in accordance with the aberration correction drivecurrent already referred to.

The focus lens 39 is constructed such that a lens containing lenselements 39 a and 39 b is held with a focus actuator 38. With such aconstruction, the focus lens 39 is adjusted in a focusing direction inaccordance with the focus drive current already mentioned to. By thefocus lens 39, the laser light is condensed on the reflecting surface ofthe optical disc 2. Specifically, the laser light that is circularlypolarized by the ¼ wavelength plate 35 is condensed by the focus lens 39and incident on a reflecting surface 2 a of the optical disc 2, througha protecting film 2 b thereof.

The focus lens 39 may consist of a single lens or a combination ofplural lenses as shown. Where the focus lens 39 is formed by thecombination of two or more lenses, each lens is not required to have alarge curvature in lens design even if a numerical aperture NA of thelens is selected to be large. This type of focus lens 39 is advantageousin that the manufacturing of the lenses is easy. Further, it is suitablefor the case of increasing the recording density and the recordingcapacity.

The optical disc 2 has a full thickness of 1.2 mm, and in its structure,a reflecting surface 2 a is formed over a substrate made ofpolycarbonate, for example. In the case of the optical disc usedexclusively for reproduction, the signal is recorded in the reflectingsurface 2 a of the optical disc. Two types of reflecting surfaces arepresent. In a first type of reflecting surface, the signal is recordedin the form of the ruggedness of the reflecting surface. In a secondtype of reflecting surface, the signal is recorded in the form of avariation of a crystal state of the reflecting surface. In the case ofthe optical disc of the recording type, the signal can be recorded inthe reflecting surface 2 a by the user. This optical disc is classifiedinto two types of optical discs, a rewritable optical disc and anon-rewritable optical disc. The protecting film 2 b for protecting thereflecting surface 2 a is also made of transparent resin, such aspolycarbonate, and its thickness is about 0.1 mm. The term “opticalsignal” in the specification involves the signal recorded in thereflecting surface.

The laser light incident on the optical disc 2 is reflected on thereflecting surface 2 a of the optical disc to be a return laser light.The return laser light travels through the optical path through whichthe laser light traveled toward the optical disc; It travels through thefocus lens 39 and the aberration correction lens 37, and then reachesthe ¼ wavelength plate 35. The return laser light passes through the ¼wavelength plate 35 to be a linearly polarized light rotated by 90° fromthe polarization direction of the laser light when it is incident on the¼ wavelength plate 35. Thereafter, the return laser light enters thepolarized light beam splitter 34 and reaches the light receiving blockD. If required, the polarized light beam splitter 34 may be substitutedby, for example, a parallel plane element serving as a half mirror.

In the embodiment, the light receiving block D is made up of acylindrical lens 41 and a light receiving part 42 including a lightreceiving device.

FIG. 4 is a diagram showing a specific example of the light receivingblock D shown in FIG. 3. As shown, the light receiving block is made upof a cylindrical lens 41 and a light receiving part 42 as a lightreceiving device. The light receiving part 42 includes a first photodiode 43 ₊ and a second photo diode 43 ⁻, which are respectively firstand second light receiving elements.

The return laser light enters the cylindrical lens 41 in the form ofparallel rays and is converged by the cylindrical lens 41. The firstphoto diode 43 _(*) as a first light receiving element receives theconverged laser light before it is imaged. Specifically, it gathersabout 50% of the received laser light, and reflects the remaining laserlight, i.e., about 50% of the received laser light. The reflected laserlight images at an focal point X, and thereafter it diverges. The laserlight then diverges and enters the second photo diode as a second lightreceiving element. The first and second photo diodes are spacedequidistantly from the focal point X. Accordingly, images that areformed on the first and second photo diodes 43 ₊ and 43 ⁻ aresymmetrical in shape with respect to a point. If the images are circularin shape, the diameters of the images are substantially equal to eachother.

FIG. 5 is a diagram showing another specific example of the lightreceiving block D shown in FIG. 3. Also in this embodiment, the lightreceiving block D of the instant example also is made up of acylindrical lens 41 and a light receiving part 42. The light receivingpart 42 includes a half-prism 44 and first and second photo diodes.

The return laser light enters the cylindrical lens 41 in the form ofparallel rays and is converged by the cylindrical lens 41. Thehalf-prism 44 permits about 50% of the received laser light to passtherethrough, and reflects the remaining laser light, i.e., about 50% ofthe received laser light. The reflected laser light is received by afirst photo diode 43 ₊ as a first light receiving element before itimages. The laser light having passed through the half-prism 44 imagesat an focal point X, and thereafter it is received by a second photodiode 43 ⁻ as a second light receiving element. If required, thehalf-prism 44 may be substituted by a parallel plane element whichpermits about 50% of the received laser light to pass therethrough, andreflects the remaining laser light, i.e., about 50% of the receivedlaser light.

In the figure, if the laser light that is reflected by the half-prism 44pass through the first photo diode 43 ₊, it will image at a point. Letthis point be a virtual focal point X′. A distance between the focalpoint X′ and the first photo diode 43 ₊ is selected to be substantiallyequal to a distance between an focal point X and the photo diode 43 ⁻.Then, in this embodiment, a distance from the first photo diode to thefocal point is equal to a distance from the second photo diode to thefocal point. Therefore, in an ideal condition that no aberration ispresent, images that are formed on the first and second photo diodes 43₊ and 43 ⁻ are symmetrical in shape with respect to a point. If theimages are circular in shape, the diameters of the images aresubstantially equal to each other.

Each photo diode generally contains a light receiving part forconverting received light into a corresponding electric signal, and aresin film covering the light receiving part. In the embodiments ofFIGS. 5 and 4, and an embodiment to be described below, the wording “adistance from the first photo diode 43 ₊ to the focal point issubstantially equal to a distance from the second photo diode 43 ⁻ tothe focal point.”, involves that a distance from the light receivingpart of the first photo diode to the focal point is substantially equalto a distance from the light receiving part of the second photo diode tothe focal point. “To slightly shift the first photo diode and/or thesecond photo diode from the focal point in order to match an image ofthe laser light irradiated on the first photo diode to an image of thelaser light irradiated on the second photo diode” is also involved inthat wording in the embodiments of FIGS. 5 and 4, and the embodiment tobe described below.

FIG. 6 is a diagram showing yet another specific example of the lightreceiving block D. In this embodiment, the light receiving block is madeup of a cylindrical lens 41 and a light receiving part 42 as a lightreceiving device. The light receiving part 42 includes a hologram 45 andfirst and second photo diodes 43 ₊ and 43 ⁻ as a set of light receivingelements. The laser light having passed through the cylindrical lens 41passes through the hologram 45 by which it is spectrally split. One ofthe split laser lights is received by a first light receiving element 43₊ before it mages. If the laser light pass through the first photo diode43 ₊, the laser light will image at a point. Let this point be a virtualfocal point X′. A distance from an focal point X to the second lightreceiving element 43 ⁻ is selected to be substantially equal to adistance from the virtual focal point X′ to a first focal point. Then,in this embodiment, as in the embodiment shown in FIG. 5, a distancefrom the first photo diode 43 ₊ to the focal point is substantiallyequal to a distance from the second photo diode 43 ⁻ to the focal point.

FIG. 7 is a diagram showing a light receiving area of a first or secondphoto diode. The first and second photo diodes 43+ and 43 ⁻ aredifferent only in location, and those photo diodes are equal instructure. Accordingly, the first and second photo diodes 43+ and 43 ⁻are generally designated by reference numeral 43. Each photo diode 43has a rectangular planar structure. The photo diode 43 contains threerectangular light receiving areas; a first light receiving area 43 a, asecond light receiving area 43 b, and a third light receiving area 43 c.Those light receiving areas 43 a, 43 b and 43 c have widths “a”, “b” and“c”, respectively.

In the light receiving blocks shown in FIGS. 4 and 5, an image Z of thelaser light reflected by the reflecting surface 2 a of the optical disc2 is circular in shape. In the instant embodiment, the diffractiongrating is ruled so that only the half of the laser light that thehologram shown in FIG. 6, for example, receives is refracted toward thelight receiving elements. Accordingly, the half of the laser light isreceived by the light receiving elements.

A central part Zc of an image Z of the laser light is formed on thefirst light receiving area 43 a. The second light receiving area 43 bdoes not receive the central part of the laser light; however, it canreceive the laser light of relatively high intensity since it adjoinsthe first light receiving area 43 a. The third light receiving area 43 creceives a fringe part of the laser light. For this reason, the thirdlight receiving area 43 c is wider than the remaining light receivingareas. If the central part of the laser light is received at the end ofthe light receiving area, there is the possibility that the photo diode43 fails to detect the central part of the laser light because ofpresence of minor manufacturing errors or the like. A width of the firstlight receiving area is selected to be larger than that of the secondlight receiving area so that the first light receiving area cansufficiently receive the central part of the laser light. The “width” ofthe light receiving area means a width of the light receiving area asviewed in the direction of a boundary line between the adjacent lightreceiving areas.

To cope with such a situation that each photo diode completely receivesthe laser light reflected by the reflecting surface 2 a of the opticaldisc 2, and an image Z formed thereon is circular in shape, what adesigner has to do is to design the photo diode such that the lightreceiving are such that second light receiving areas are located on bothsides of a first light receiving area, and third light receiving areasare each located on the other side of the first light receiving areawith respect to the related second light receiving area. That is, thephoto diode is configured such that the first light receiving area islocated between the second light receiving areas, and the sum of thefirst and second light receiving areas is located between the thirdlight receiving areas.

In FIG. 7, reference numeral 2 c indicates a track direction 2 c of theoptical disc 2. In the instant embodiment, the photo diode 43 isarranged such that the track-directional component of the laser lightreflected by the reflecting surface 2 a of the optical disc 2 isvertical to a boundary line between the first and second light receivingareas 43 a and 43 b, and a boundary line between the second and thirdlight receiving areas 43 b and 43 c. The reason why the photo diode isso arranged is that it is necessary to suppress adverse effects whichwill be produced when the focus lens 39 shifts in a direction (radialdirection) vertical to the track of the optical disc 2 by the follow-upaction of the tracking-servo. Specifically, if the track becomeseccentric and the focus lens 39 shifts, the image Z of the reflectedlaser light shifts to an image Z′ while moving in a direction verticalto the track direction 2 c. Therefore, if the focus lens 39 shifts, avariation of the amount of light received by each light receiving areais minimized or reduced to zero since the photo diode 43 is arranged tobe vertical to the boundary line between the adjacent light receivingareas. As a result, the output signal of each light receiving area islittle affected.

FIG. 8 is a diagram showing in detail the light receiving part 42 in theembodiment of FIG. 6. In FIG. 8, the hologram 45 is segmented into threelight receiving areas 45 a, 45 b and 45 c. For the hologram shown inFIG. 6, segmenting of the hologram as in the case of the FIG. 8 is notessential in the invention, however. The laser light diffracted at asemi-circular area 45 a of the hologram 45 is received by a first photodiode 43 ₊ and a second photo diode 43 ⁻. A relation between an image Z₊appearing on the first photo diode 43 ₊ and an image Z⁻ appearing on thesecond photo diode 43 ⁻ corresponds to a relation between the plus1st-order light before the light images and the minus 1st-order lightbefore the laser light images. Therefore, those images are shaped to besymmetrical in shape with respect to a point. Therefore, the first photodiode 43 ₊ and the second photo diode 43 ⁻ are arranged to besymmetrical with respect to a point located between those photo diodesas a point. The word “point-symmetrical” means that the two photo diodesare arranged to be symmetrical with respect to a point when viewed fromabove. A step in the light incident direction or the like caused whenthe photo diodes receive the plus and minus 1st-order lights from thefocal point at equal distances, is considered to be within an error.

Each photo diode is arranged such that a boundary line between the firstlight receiving area 43 a ₊ (43 a ⁻) and the second light receiving area43 b ₊ (43 b ⁻) and a boundary line between the second light receivingarea 43 b ₊ (43 b ⁻) and the third light receiving area 43 c ₊ (43 c ⁻)are substantially vertical to a direction of the diffraction grating ofthe semi-circular area 45 a of the hologram 45. The reason why thosediodes are so arranged is that it is necessary to suppress adverseeffects which will be produced when the wavelength of the laser lightvaries by temperature variation of the related device. Specifically,when the wavelength of the laser light varies by the temperaturevariation or the like, an angle at which the laser light is diffractedby the hologram 45 changes. By this angle change, images Z₊ and Z⁻appearing on the first and second photo diodes shift in a directionsubstantially vertical to the grating to be images Z′₊ and Z′⁻ asindicated by dotted lines, respectively. By arranging the photo diodes43 ₊ and 43 ⁻ such that the boundary lines each between the adjacentlight receiving areas are vertical to the direction of the grating ofthe semi-circular area of the hologram, the adverse effect by thewavelength variation of the laser light owing to the temperaturevariation is lessened. Sometimes one or some of the parallel lines ofthe grating are slightly bent. In the specification, such slight bendingof the lines of the grating is neglected, and an average of thedirections of the lines of the grating is used for the direction of thelines of the grating.

Also in the instant embodiment, it is preferable that each of the firstand second photo diodes 43 ₊ and 43 ⁻ is arranged such that, as shown inFIG. 7, a track directional component of the laser light reflected bythe optical disc 2 is vertical in orientation to a boundary line betweenthe first light receiving area 43 a ₊ (43 a ⁻) and the second lightreceiving area 43 b ₊ (43 b ⁻) and a boundary line between the secondlight receiving area 43 b ₊ (43 b ⁻) and the third light receiving area43 c ₊ (43 c ⁻).

FIG. 9 is a diagram showing in model form preconditions for thecomputation of an aberration amount and the like by using a hologramconsisting of three segmental areas, which is described referring toFIG. 8. In the illustrated case, three beams of different frequencies, amain beam, a first sub-beam and a second sub-beam, are used for thelaser light. Further, there are illustrated images formed on the photodiodes by the laser light diffracted by three light receiving areas 45a, 45 b and 45 c of the hologram 45.

The plus 1st-order light of the main beam diffracted by thesemi-circular area 45 a is received by the first photo diode 43 ₊, andthe minus 1st-order light of the main beam is received by the secondphoto diode 43 ⁻. In an image Z₊ received by the first aph 43 ₊, asegmental image a₊ is formed by the laser light received by the firstlight receiving area 43 a ₊, a segmental image b₊ is formed by the laserlight received by the second light receiving area 43 b ₊, and asegmental image c₊ is formed by the laser light received by the thirdlight receiving area 43 c ₊. Of an image Z⁻ received by the second photodiode 43 ⁻, a segmental image a⁻ is formed by the laser light receivedby the first light receiving area 43 a ⁻, a segmental image b is formedby the laser light received by the second light receiving area 43 b ⁻,and a segmental image c⁻ is formed by the laser light received by thethird light receiving area 43 c ⁻.

The main beam, the first sub-beam, and the second sub-beam are arrayedat a spatial interval of 120 μm in the embodiment. Accordingly, when thelaser light reaches the light receiving elements 43 ₊ and 43 ⁻, the mainbeam, the first sub-beam and the second sub-beam are spaced apart fromone another by a distance of 120 μm. Design must be made such thatneither of those photo diodes receives the first sub-beam and the secondsub-beam that are diffracted by the area 45 a. Therefore, the width ofeach of the light receiving elements 43 ₊ and 43 ⁻ is selected to be 120μm, preferably 100 μm.

The laser light diffracted at the area 45 b is received by other relateddiodes, which are different from the first and second light receivingelements 43 ₊ and 43 ⁻. The main beam is diffracted and split by thearea 45 b, and one of the split beams is received as the plus 1st-orderlight by one of the photo diodes before it images, and the other isreceived as the minus 1st-order light by the other photo diode after itimages. The same thing is correspondingly applied to the first andsecond sub-beams spectrally split by the area 45 c. One of the splitfirst sub-beams is received as the plus 1st-order light by one of therelated photo diodes before it images, and the other is received as theminus 1st-order light by the other diode after it images. One of thesplit second sub-beams is received as the plus 1st-order light by one ofthe related photo diodes before it images, and the other is received asthe minus 1st-order light by the other diode after it images.

Under such conditions, images of the plus 1st-order laser lights of themain beam, and the first and second sub-beams that are diffracted at thearea 45 b successively become images bp₊, bq₊, and br₊ on the photodiode, as shown. The minus laser lights of those beams successively formimages bp⁻, bq⁻, and br⁻ on the diode.

Similarly, the laser light that is diffracted by the area 45 c is alsoreceived by other diodes which are different from the first and secondphoto diodes 43 ₊ and 43 ⁻ and also different from the diodes forreceiving the laser light diffracted by the area 45 b. In this case, themain beam is diffracted and split by the area 45 c, and one of the splitbeams is received as the plus 1st-order light by one of the relatedphoto diodes before it images, and the other is received as the minus1st-order light by the other photo diode after it images. The same thingis correspondingly applied to the first and second sub-beams spectrallysplit by the area 45 c. One of the split first sub-beams is received asthe plus 1st-order light by one of the related photo diodes before itmages, and the other is received as the minus 1st-order light by theother diode after it images. One of the split second sub-beams isreceived as the plus 1st-order light by one of the related photo diodesbefore it images, and the other is received as the minus 1st-order lightby the other diode after it images.

Under such conditions, images of the plus 1st-order laser lights of themain beam, and the first and second sub-beams that are diffracted at thearea 45 c successively become images cp₊, cq₊, and cr₊ on the photodiode, as shown. The minus laser lights of those beams successively formimages cp⁻, cq⁻, and cr⁻ on the diode.

In combinations of the images bp₊, bq₊, and br₊ and the images bp⁻, bq⁻,and br⁻, and the images cp₊, cq₊, and cr₊ and the images cp⁻, cq⁻, andcr⁻, a distance from a virtual focal point at which the plus 1st-orderlight images if it pass through the photo diode to the photo diode isselected to be substantially equal to a distance from an focal point ofthe minus 1st-order light to the photo diode. Therefore, in eachcombination, under an ideal condition that no aberration is present,those images are symmetrical in shape with each other.

In the instant embodiment, the laser lights spectrally split by theareas 45 b and 45 c are also received by the photo diodes before andafter the laser lights image, for ease of understanding. The areas 45 band 45 c are provided for detecting a tracking error signal. Therefore,there is no need of receiving the plus and minus 1st-order laser lightsbefore and after the laser lights image. Accordingly, design may be madeso as to avoid the deterioration of the photo diodes by heating at thefocal points and to avoid the overlapping of the main beam with thesub-beams on the light receiving elements

Methods for computing an aberration amount, a focus error quantity andthe like by using the output signals of the photo diodes shown in FIG.9, will be described below. The symbols are used for indicating theimages shown in FIG. 9 and the output signals from the photo diodes onwhich the images are formed. In FIG. 2, an electrical signal output fromthe optical pickup 30 is input to the focus error detecting circuit 8,the tracking error detecting circuit 9, and the aberration amountdetecting circuit 10. Upon receipt of the signal, those circuitscalculate a focus error signal, a tracking error signal, and anaberration amount signal.

The aberration amount detecting circuit 10 is capable of detecting anaberration amount AB by using only an output signal of the first photodiode 43 ₊ and an output signal of the second 43 ⁻. Mathematically,AB=a₊−a⁻. This equation for computing the aberration amount is verysimple since what one has to do for obtaining the aberration amount AB,is to merely compute a difference between the output signal from thefirst area of the first photo diode 43 ₊ and the output signal of thesecond photo diode 43 ⁻. Further, any of the following computing methodsmay be used:AB=(a ₊ +b ⁻)−(b ₊ +a ⁻),  1)AB=(a ₊ +b ⁻ +c ⁻)−(a ⁻ +b ₊ +c ₊),  2)AB=(a ₊ +b ₊ +c ⁻)−(a ⁻ +b ⁻ +c ₊),  3)AB=(a ₊ +b ⁻ +c ₊)−(a ⁻ +b ₊ +c ⁻),  4)AB=(a ₊ +b ₊)−(a ⁻ +b ⁻)  5)

In the instant embodiment, the laser light received by the first andsecond photo diodes 43 ₊ and 43 ⁻ is shaped to be semi-circular by useof the hologram 45 in order to correct the tracking error too. It isevident that the laser light being circular in cross section, which isformed by entirely using the laser light, maybe used for computing theaberration amount, the focus error and the like.

The focus error signal may be computed by using only the output signalsof the photo diodes, viz., output signals from the first and secondphoto diodes 43 ₊ an 43 ⁻. Mathematically, FO=a₊+a⁻. This equation, likethe equation for computing the aberration correction amount, is verysimple. And the aberration amount can be computed in a very simplemanner. Other computing methods may be enumerated as in the case ofcomputing the aberration correction amount.FO=(a ₊ +b ⁻)−(b ₊ +a ⁻),  1)FO=(a ₊ +b ⁻ +c ⁻)−(a ⁻ +b ₊ +c ₊),  2)FO=(a ₊ +b ₊ +c ⁻)−(a ⁻ +b ⁻ +c ₊),  3)FO=(a ₊ +b ⁻ +c ₊)−(a ⁻ +b ₊ +c ⁻),  4)FO=(a ₊ +b ₊)−(a ⁻ +b ⁻)  5)

The tracking error signal can be computed by using the followingequationtr=(bp ₊ +bp ⁻ −cp ₊ −cp ⁻)−k(bq ₊ +bq ⁻ +br ⁻ −cq ₊ −cq ⁻ −cr ₊ −cr ⁻)where “k” is constant. The tracking signal may also be computed by usingthe following equationtr=(bq ₊ +bq ⁻ −cq ₊ −cq ⁻)−(br ₊ +br ⁻ +cr ₊ +cr ⁻)

The sum of all the signals by the plus and minus 1st-order lights may beused for the signals output as data signals.

The focus error signal, the tracking error signal and the aberrationamount signal, which are thus generated, are input to the servoprocessing circuit 16. In turn the servo processing circuit 16 outputsservo signals to the focus driver 22, the tracking driver 23, and theaberration correction driver 24 which in turn generate a focus drivecurrent, a tracking drive current, and an aberration amount correctiondrive current. And, as shown in FIG. 3, the focus drive current drivesthe focus actuator 38 to adjust a position of the focus lens 39. Thetracking is corrected by the tracking drive current. The aberrationamount correction drive current drives the aberration correctionactuator 36 to adjust a position of the aberration correction lens.

In this way, the focus and tracking adjustments are performed, and theaberration is reduced. In another aberration reducing process, theaberration is corrected at once when or before a signal is read out ofthe optical disc 2, and subsequently no further aberration correction isperformed. In still another aberration reducing process, the aberrationis corrected continuously or successively several times.

FIG. 10 is a diagram showing another optical pickup as one form of anoptical signal detecting device constructed according to the invention.Also in the instant embodiment, an optical pickup 30′ is operable forboth information reproducing and recording purposes. In FIG. 10, thesame reference numerals are given to components having the samefunctions as those of the components in the optical pickup 30 shown inFIG. 3, and their detailed description is omitted.

A semiconductor laser device 31 is a semiconductor laser device in theinstant optical pickup. The laser light emitted from the semiconductorlaser device 31 is collimated by a collimator lens 32, and thecollimated one enters the beam forming prism 33 which in turn reshapesthe laser light into the laser light having a substantially completeround in cross section. Such a construction of the optical pickup issubstantially the same as of the optical pickup shown in FIG. 3.

After passing through the beam forming prism 33, the laser light entersa first polarized light beam splitter 34 a as a sort of half prism. Apower-monitor photo diode (not shown) may be provided as in the case ofFIG. 3. In this case, about 10% of the laser light emitted from thelight source is reflected toward the power-monitor photo diode, and thisdiode monitors an intensity of laser light emitted from thesemiconductor laser device 31.

The laser light emanating from the first polarized light beam splitter34 a passes through a second polarized light beam splitter 34 b. Thepower-monitor photo diode may be positioned in association with thesecond polarized light beam splitter 34 b in place of the firstpolarized light beam splitter 34 a. After passing through the secondpolarized light beam splitter 34 b, the laser light passes through the ¼wavelength plate 35, the aberration correction lens 37 and the focuslens 39. The operation of the optical pickup thus far stated issubstantially the same as of the optical pickup shown in FIG. 3.

As in the case shown in FIG. 3, the aberration correction lens 37 isheld with an aberration correction actuator 36 containing a combinationof a coil and a magnet as a major part, whereby those components formaberration correction means. Similarly, the focus lens 39 is held with afocus actuator 38 containing a combination of a coil and a magnet as amajor part, whereby those components form another aberration correctionmeans. Those lenses are adjusted in position by the aberrationcorrection drive current and the focus drive current. In the opticalpickup, the tracking is also adjusted by the tacking drive current fedfrom the tracking driver 23.

The laser light having passed through the focus lens 39 is incident on areflecting surface 2 a of the optical disc 2, through a protecting film2 b thereof. The laser light incident on the optical disc 2 is reflectedon the reflecting layer of the optical disc to be a return laser light.The return laser light travels through the optical path through whichthe laser light traveled toward the optical disc; It travels through thefocus lens 39 and the aberration correction lens 37, and then reachesthe ¼ wavelength plate 35. The return laser light passes through the ¼wavelength plate 35 to be a linearly polarized light rotated by 90° fromthe polarization direction of the laser light when it is incident on the¼ wavelength plate 35. Thereafter, the return laser light enters thesecond polarized light beam splitter 34 b.

When receiving the return laser light, the second polarized light beamsplitter 34 b reflects about 45% of the return laser light and allowsthe remaining return laser light, which is 55% of the return laserlight, to pass therethrough. The laser light having passed through thesecond polarized light beam splitter 34 b enters the first polarizedlight beam splitter 34 a. When receiving the return laser light, thefirst polarized light beam splitter 34 a reflects about 82% of thereturn laser light and permits the remaining laser light, which is 18%of the return laser light, to pass therethrough.

The lights reflected by the second and first polarized light beamsplitters 34 b and 34 a are incident on a second cylindrical lens 41 ⁻and a first cylindrical lens 41 ₊, which in turn condense the lights bytheir refraction.

The laser light that is reflected by the second polarized light beamsplitter 34 b and condensed by the second cylindrical lens 41 ⁻ imagesat the focal point X, and diverges, and is received by the second lightreceiving element 43 ⁻. The laser light that is reflected by the firstpolarized light beam splitter 34 a and condensed by the firstcylindrical lens 41 ₊ 0 is received by the first light receiving element43 ₊ as the first light receiving element before it images at the focalpoint. The constructions of the first and second photo diodes 43 ₊ and43 ⁻ are substantially the same as of the corresponding photo diodesshown in FIG. 7.

If the laser light pass through the first photo diode 43 ₊, the laserlight that is condensed by the first cylindrical lens 41 ₊ will image ata point. Let this point be a virtual focal point X′. The first andsecond photo diodes 43 ₊ and 43 ⁻ are arranged so that a distance fromthe virtual focal point X′ to the first photo diode 43 ₊ is selected tobe substantially equal to a distance from the focal point X to thesecond light receiving element 43 ⁻. With such a diode arrangement, theimages received by the laser light received by the first and secondphoto diodes 43 ₊ and 43 ⁻ are symmetrical in shape with each other inan ideal condition where no aberration is present.

In the instant embodiment, the two polarized light beam splitters, whichare each a sort of half prism, are used for means for guiding the laserlight to the photo diodes. If required, one or both of the polarizedlight beam splitters may be substituted by a parallel plane element,such as a half mirror, which permits part of light to pass therethroughand reflects the remaining part of light.

Also in the instant embodiment, it is preferable that each of the firstand second photo diodes 43 ₊ and 43 ⁻ is arranged such that, as shown inFIG. 7, a track directional component of the laser light reflected bythe optical disc 2 is vertical in orientation to a boundary line betweenthe first light receiving area 43 a and the second light receiving area43 b and a boundary line between the second light receiving area 43 band the third light receiving area 43 c.

FIG. 11 is a diagram showing still another optical pickup as one form ofan optical signal detecting device constructed according to theinvention. In the instant embodiment, the photo diodes are contained ina semiconductor laser device as a light source. Also in the instantembodiment, an optical pickup 30″ is operable for both informationreproducing and recording purposes. In FIG. 10, the same referencenumerals are given to components having the same functions as those ofthe components in the optical pickups 30 and 30′ shown in FIGS. 3 and10, and their detailed description is omitted.

Laser light emitted from a semiconductor laser device 50 capable ofemitting and receiving light is collimated by a collimator lens 32, andenters a beam forming prism 33 where the laser light is reshaped intolaser light having a complete round in cross section. Such aconstruction of the optical pickup is substantially the same as of theoptical pickups 30 ad 30′ shown in FIG. 3.

After passing through the beam forming prism 33, the laser light passesthrough the ¼ wavelength plate 35, the aberration correction lens 37 andthe focus lens 39. The operation of the optical pickup thus far statedis also substantially the same as of the optical pickups shown in FIGS.3 and 10.

As in the case shown in FIGS. 3 and 10, the aberration correction lens37 is held with an aberration correction actuator 36 containing acombination of a coil and a magnet as a major part. Similarly, the focuslens 39 is held with a focus actuator 38 containing a combination of acoil and a magnet as a major part. Those lenses are adjusted in positionby the aberration correction drive current fed from the aberrationcorrection driver 24 and the focus drive current from the focus driver22. In the optical pickup, the tracking is also adjusted by the tackingdrive current fed from the tracking driver 23.

The laser light having passed through the focus lens 39 is incident on areflecting surface 2 a of the optical disc 2, through a protecting film2 b thereof. The laser light incident on the optical disc 2 is reflectedon the reflecting layer of the optical disc to be a return laser light.The return laser light travels through the optical path through whichthe laser light traveled toward the optical disc; It travels through thefocus lens 39 and the aberration correction lens 37, and then reachesthe ¼ wavelength plate 35. The operation of the optical pickup thus farstated is also substantially the same as of the optical pickups shown inFIGS. 3 and 10.

The return laser light having passed through the ¼ wavelength plate 35passes through the beam forming prism 33 again, is collimated by thecollimator lens 32, and is received in the semiconductor laser device 50capable of emitting and receiving light.

FIG. 12 is a model diagram showing in detail a semiconductor laserdevice 50 capable of emitting and receiving light shown in FIG. 11. Inthe semiconductor laser device 50, a heat sink 56 is integrally providedon a disk-like base 51 made of insulating material. A semiconductorlaser element 55 as a light emitting element is attached to the heatsink 56 in a state that a laser oscillating surface thereof is directedupward. Laser light emitted upward from the light emitting element 55 isirradiated on the optical disc 2 as shown in FIG. 11. A power-monitorphoto diode (not shown) may be provided at a position where laser lightgenerated by the light emitting element 55 can be received. In thiscase, an output power of the laser light generated by the light emittingelement 55 is adjusted in accordance with detection result by thepower-monitor photo diode.

A first photo diode 57 ₊ as a first light receiving element and a secondphoto diode 57 ⁻ as a second light receiving element, both diodes beingfor signal detection, are installed at positions where the laser lightthat is emitted from the light emitting element 55 and reflected by theoptical disc can be received. The base 51 includes a plurality of leadwires 58 which pass through the base 51. The ends of the lead wires 58appearing on the upper surface of the base 51 are electrically connectedto the light emitting element 55 and the first and second photo diodes57 ₊ and 57 ⁻ by wire bonding (not shown).

The heat sink 56, the light emitting element 55, the first and secondphoto diodes 57 ₊ and 57 ⁻, and the like which are on the base 51, arecovered with an housing 52 bonded to the base 51. The housing 52 iscylindrical in shape. A bottom of the housing 52 is opened and anopening part 54 is formed in the top surface of the housing 52. Theopening part 54 of the top surface is closed with a hologram 53 mountedon the top surface of the housing 52.

Normally, the inside of the housing 52 is formed with a transparentoptical member. Sometimes, the housing is filled with a transparentfluid material, instead. The fluid material has such a transparency asto allow laser light having a wavelength generated by the light emittingelement 55 to pass therethrough. Examples of such materials are siliconeoil, fluorine inert liquid, and mineral oil.

In the optical pickup 30 thus constructed, laser light generated by thelight emitting element 55 passes through the fluid material and thehologram 53 and reaches the optical disc.

The laser light that is reflected by the optical disc and collimated bythe collimator lens 32 shown in FIG. 11 is spectrally split by thehologram 53, and reaches the first pulse signals before it images. Thelaser light images at an focal point X and diverges and in this state itreaches the second photo diode. If the laser light pass through thefirst photo diode 57 ₊, it will image at a point. Let this point be avirtual focal point X′. The first and second photo diodes 57 ₊ and 57 ⁻are arranged such that a distance between the focal point X′ and thefirst photo diode 57 ₊ is selected to be substantially equal to adistance between an focal point X and the second photo diode 57 ⁻.

The first and second photo diodes 57 ₊ and 57 ⁻ are similar inconstruction to those shown in FIG. 7. Each photo diode is segmentedinto two types of sectional areas. And, one type of sectional area islocated between the other type of sectional areas. It is preferable thata boundary line between the first and second light receiving areas isoriented to be vertical to the parallel lines of the grating of thehologram 53.

Also in the instant embodiment, it is preferable that each of the firstand second photo diodes 57 ₊ and 57 ⁻ is arranged such that, as shown inFIG. 7, a track directional component of the laser light reflected bythe optical disc 2 is vertical in orientation to a boundary line betweenthe first light receiving area and the second light receiving area and aboundary line between the second light receiving area and the thirdlight receiving area.

In all of the embodiments mentioned above, the device including theoptical disc as the light receiving device, the aberration amountdetecting circuit, the aberration control circuit, and the aberrationcorrection driver may be operated as an aberration amount detectingdevice.

<Embodiment>

The present invention will be described in detail by using a specificexample. In the example, a light receiving device similar to thatdescribed in connection with FIGS. 6 and 8 was used for the photo diodesas light receiving devices. In the description, discussion will be givento only the semi-circular area 45 a in FIG. 8, exclusive of theremaining areas 45 b and 45 c. Light receiving elements as describedreferring to FIG. 7 were used for the photo diodes. A wavelength λ oflaser light was 405 nm; λ=405 nm. A numerical aperture NA for theincoming optical path was 0.1; NA=0.1.

A η axis direction as a vertical direction of each image is coincidentwith a boundary line between the adjacent areas of those first o thirdareas. Accordingly, where the direction of the boundary line is verticalto the track direction 2 c of the optical disc 2, a component of thetrack direction 2 c shown in FIG. 6 is contained in a ξ axis directionas a lateral direction of each image. Further, where the parallel linesof the diffraction grating of the hologram are vertical to the boundarydirection, a component of the grating is contained in the ξ axisdirection as a lateral direction of each image.

FIG. 13 is a diagram showing images appearing on a light receivingelement and distributions of light intensity along a lateral axispassing through the center of the image when a protecting layer of theoptical disc is thinner than a predetermined thickness of 0.1 mm by 20μm.

A of FIG. 13 shows an image and a light intensity distribution at apoint spaced apart from an focal point by −500λ=0.203 mm. As seen fromthe distribution graph, a light intensity curve sharply rises in thevicinity of ξ=0 μm. It is observed that a small peak of the curveappears in the vicinity of ξ=80 μm too.

B of FIG. 13 shows an image and a light intensity distribution at apoint spaced apart from an focal point by −1000λ=0.405 mm. As seen fromthe distribution graph, a light intensity curve sharply rises in thevicinity of ξ=35 μm. This peak is lower in steepness than that in A ofFIG. 13. It is observed that small peaks appear in the curve in a regionof about 70 μm (=ξ) and longer.

C of FIG. 13 shows an image and a light intensity distribution at apoint spaced apart from an focal point by −2000λ=0.81 mm. As seen alsofrom the graph, peaks appear in the vicinity of ξ=20 μm and ξ=40 μm.Peaks appearing in the vicinity of ξ=60 μm and ξ=90 μm are somewhathigher than those in the vicinity of ξ=20 μm and ξ=40 μm.

Images at points distanced from the focal point by +500λ, +1000λ, and+2000λ are those obtained by turning back the images A, B and Cpoint-symmetrically. The same thing is true for the graphs. The scalesizes of the ordinates of the graphs A, B and C are different from eachother.

FIG. 14 is a diagram showing images appearing on a light receivingelement, and distributions of light intensity along a lateral axispassing through the center of the image when a protecting layer of theoptical disc has a predetermined thickness of 0.1 mm, viz., when noerror is contained in the thickness dimension. In this case, noaberration occurs except the defocusing.

A of FIG. 14 shows an image and a light intensity distribution at apoint spaced apart from an focal point by −500λ=0.203 mm. As seen fromthe distribution graph, a peak of the curve appears in the vicinity ofξ=5 μm.

B of FIG. 14 shows an image and a light intensity distribution at apoint spaced apart from an focal point by −1000λ=0.405 mm. As seen fromthe distribution graph, peaks of the curve appears in the vicinity ofξ=10 μm, ξ=20 μm, and ξ=30 μm. Further, the curve peaks in the vicinityof ξ=80 μm.

C of FIG. 14 shows an image and a light intensity distribution at apoint spaced apart from an focal point by −2000λ=0.81 mm. As seen fromthe distribution graph, gentle peaks of the curve appears in a regionroughly ranging from ξ=10 μm to ξ=70 μm. Further, the curve sharplypeaks in the vicinity of ξ=110 μm.

Images at points distanced from the focal point by +500λ, +1000λ, and+2000λ are those obtained by turning back the images A, B and Cpoint-symmetrically. The same thing is true for the graphs. The scalesizes of the ordinates of the graphs A, B and C are different from eachother.

FIG. 15 is a diagram showing images appearing on a light receivingelement, and distributions of light intensity along a lateral axispassing through the center of the image when a protecting layer of theoptical disc is thinner than a predetermined thickness of 0.6 mm by 20μm.

A of FIG. 15 shows an image and a light intensity distribution at apoint spaced apart from an focal point by −500λ=0.203 mm. As seen fromthe distribution graph, the curve sharply rises in the vicinity of ξ=5μm. Further, the curve peaks in the vicinity of ξ=80 μm too.

B of FIG. 15 shows an image and a light intensity distribution at apoint spaced apart from an focal point by −1000λ=0.405 mm. As seen alsofrom the distribution graph, a large peak appears in the vicinity ofξ=10 μm. This peak is lower in steepness than that in A of FIG. 15. Arelatively large peak appears in the vicinity of ξ=80 μm, and a peak isobserved also in the vicinity of ξ=100 μm.

C of FIG. 15 shows an image and a light intensity distribution at apoint spaced apart from an focal point by −2000λ=0.81 mm. As seen alsofrom the distribution graph, a gentle peak appears in the vicinity ofξ=20 μm, and a steep and strong peak appears in the vicinity of ξ=110μm.

Images at points distanced from the focal point by +500λ, +1000λ, and+2000μ are those obtained by turning back the images A, B and Cpoint-symmetrically. The same thing is true for the graphs. The scalesizes of the ordinates of the graphs A, B and C are different from eachother.

FIGS. 16 and 17 are graphs showing variations of signals with respect tothickness errors of a protecting film, which are computed by use of aplurality of formulae, with the light receiving elements 43 ₊ and 43 ⁻described in connection with FIG. 8 being used. In this graph, anintensity distribution of a spot obtained by converting a thicknesserror to a corresponding spherical aberration amount, was computedthrough a simulation based on the scalar diffraction theory. And, anamount of light landing on each light receiving area was computed,whereby a signal output was obtained.

In the computations, a numerical aperture NA was 0.1 and the wavelengthwas 405 mm. In FIG. 16, a distance α from an focal point X to each ofthe light receiving elements 43 ₊ and 43 ⁻ was 0.162 mm (400λ). In FIG.17, a distance α from an focal point X to each of the light receivingelements 43 ₊ and 43 ⁻ was 0.243 mm (600λ).

As seen from the graph of FIG. 16, a curve represented byAB=(a₊+b₊+c⁻)−(a⁻+b⁻−c₊) indicated by a solid line little varies withrespect to a protecting layer thickness error. Accordingly, in case ofthis condition, if this formula is used for detecting a focus error, avariation of the spherical aberration amount is little affected. And,hence, little interference occurs in the feedback control. The curvesrepresented by AB=(a₊+b⁻)−(a⁻+b₊), AB=(a₊+b⁻+c⁻)−(a⁻+b₊−c₊), andAB=(a₊+b⁻+c₊)−(a⁻+b₊+c⁻) show that the signal output change graduallyincreases with respect to the protecting layer thickness error.Accordingly, it is seen that when those formulae are used, an aberrationamount can be detected satisfactorily. Also when the detection formulaeof AB=(a₊+b₊)−(a⁻+b⁻) and AB=a₊−a⁻ were used, it was confirmed that theaberration detection was satisfactory, although not shown. In practicalstage, it is suggestible to select appropriate formulae from those oneswhile considering operation circuit elements that can be used in thesystem.

In the graph of FIG. 17, a curve represented by AB=(a₊+b₊+c⁻)−(a⁻+b⁻−c₊)indicated by a solid line varies with respect to a protecting layerthickness error. However, the curve varies in a direction that isopposite to a variation direction of each of the remaining curves in aregion where the total width is small. When a thickness error of theprotecting layer exceeds a range of ±20 μm, the output signal decreasesin its level. Accordingly, where the aberration amount exceeds apredetermined value of it, there is the possibility that the protectinglayer thickness is erroneously recognized. Accordingly, use of thiscondition is not appropriate to the detection of the aberration amountAB. The curves represented by AB=(a₊+b⁻)−(b₊+a⁻),AB=(a₊+b⁻+c⁻)−(a⁻+b₊−c₊), and AB=(a₊+b⁻+c₊)−(a⁻+b₊+c⁻) show that thesignal output change gradually increases with respect to the protectinglayer thickness error. Accordingly, it is seen that when those formulaeare used, an aberration amount can be detected satisfactorily. Thus,where a set of light receiving elements each having three segmentalareas are arranged at fixed spatial intervals over a range from thefocal point X to the photo diode 43, the aberration amount and the focuserror can be detected by the detection formulae appropriately selectedaccording to various factors, such as the distance from the focal pointto the light receiving element.

FIG. 18 is a graph showing variations of the focus correction amount ofFO=(a₊+b₊+c⁻)−(a⁻+b⁻+c₊) with respect to a distance α from the focalpoint X to the photo diode 43. In the graph, the total width “a+b” ofthe first and second light receiving areas is appropriately varied. Inthe instant embodiment, the whole width of the light receiving elementis fixed at 110 μm. Therefore, if the total width “a+b” of the first andsecond light receiving areas shown in FIG. 7 varies, the width of thethird light receiving area also varies correspondingly. The graph showsthat within a range from 0.1 to 0.5 mm of the distance from the focalpoint to each light receiving element, some curves exhibit greatvariations of the signal output value. When the distance from the focalpoint to each light receiving element exceeds 0.5 mm, the signal outputvalue little varies for any of the total width “a+b” of the first andsecond light receiving areas.

In this instance, the focus correction amount isFO=(a₊+b₊+c⁻)−(a⁻+b⁻+c₊). The signal computing methods may beappropriately chosen according to design factors, such as an opticalsystem and the width of the light receiving element. The computingresults of the instant formula are typical ones. Other formulae willproduce computing results resembling those by the formulae describedabove.

FIG. 19 shows variations of focus correction signals each represented byFO=(a₊+b⁻+c⁻)−(a⁻+b⁻+c₊) with respect to the total width of a width “a”of the first light receiving area 43 a of the photo diode as the lightreceiving element described referring to FIG. 7 and a width “b” of thesecond light receiving area 43 b of the same. In the graph,appropriately selected distances a from the focal point to the lightreceiving element were used.

As seen from the graph, in a region where the light receiving elementwidth “a+b” exceeds 50 μm, no signal output is produced for any of theselected distances α from the focal point to the light receivingelement. In other words, within a region where the total width “a+b” is20 μm to 50 μm, the signal output is produced by appropriately selectingthe distance a from the focal point to the light receiving element.

Also in the graphs shown in FIGS. 18 and 19, an intensity distributionof the spot was computed through a simulation based on the scalardiffraction theory, and an amount of light landing on each lightreceiving area was computed, whereby a signal output was obtained. Inthe computations, a numerical aperture NA was 0.1 and the wavelength was405 mm.

As seen from the foregoing description, in the invention, the lightreceiving elements each having three light receiving areas are disposedat positions equidistantly spaced from the focal point. Computations maybe appropriately performed by using the output signals from those lightreceiving areas. Hence, the aberration amount and the focus correctionamount can be computed based on them. In particular, the light receivingelements are not positioned at the focal point, so that light may belanded on each light receiving area. As a result, it is easy to positionand orient the light receiving elements.

1. A light receiving device having a condenser which generates condensedlight, comprising: a first light receiving element, which receives thecondensed light before the condensed light images; and a second lightreceiving element, which receives the condensed light after thecondensed light images, wherein the first light receiving element andsecond light receiving element are disposed at positions equidistantlyspaced from a focal point of the condensed light, and generateelectrical signals based on light received by the light receivingelements, wherein each of the first light receiving element and secondlight receiving element includes: a first light receiving area, whichreceives a first portion of the condensed light which includes a opticalaxis of the condensed light; a second light receiving area, whichreceives a second portion of the condensed light which is locatedoutside of the first portion of the condensed light; and a third lightreceiving area, which receives a third portion of the condensed lightwhich is located outside of the second portion of the condensed light.2. The light receiving device according to claim 1, wherein the firstlight receiving element and the second light receiving element aresymmetrical with respect to a point located between the first lightreceiving element and the second light receiving element.
 3. The lightreceiving device according to claim 1, wherein a width of the firstlight receiving area is larger than a width of the second lightreceiving area.
 4. The light receiving device according to claim 1,wherein a width of the third light receiving area is larger than a widthof each of the first and second light receiving areas.
 5. The lightreceiving device according to claims 1, wherein a total width of thewidths of the first and second light receiving areas is 20 to 50 μm. 6.The light receiving device according to claim 5, wherein each width ofthe first light receiving area and the second light receiving area are10 to 30 μm.
 7. The light receiving device according to claim 5, whereinthe width of the third light receiving area is 40 to 180 μm.
 8. Thelight receiving device according to claim 1, wherein the first lightreceiving element and second light receiving element are located atpositions spaced apart from the focal point of the light by a distanceof 0.1 to 0.5 mm.
 9. The light receiving device according to claim 1,wherein the first light receiving element receives one of lightsspectrally split by a splitter, and the second light receiving elementreceives the other split light.
 10. The light receiving device accordingto claim 9, wherein the splitter is at least one of a half prism, aparallel plane element, and a hologram element.
 11. The light receivingdevice according to claim 10, wherein in a case where a hologram is usedas the splitter, a boundary line between the first light receiving areaand second light receiving area, and a boundary line between the secondlight receiving area and third light receiving area, are substantiallyvertical to a grating of the hologram.
 12. The light receiving deviceaccording to claim 1, wherein the condensed light is light reflectedfrom an optical recording medium.
 13. The light receiving deviceaccording to claim 12, wherein a boundary line between the first lightreceiving area and second light receiving area, and a boundary linebetween the second light receiving area and third light receiving area,are substantially vertical to a direction of a component of thereflected light in a track direction of the optical recording medium.14. The light receiving device according to claim 1, further comprising:an aberration correction driver, which generates an aberrationcorrection drive current based on the output signals of the first lightreceiving element and second light receiving element, an aberrationcorrector, which corrects a quantity of aberration of the lightreflected from the optical recording medium in accordance with theaberration correction drive current.
 15. A light detecting devicecomprising: a first light receiving element, which receives thecondensed light before the condensed light images; and a second lightreceiving element, which receives the condensed light after thecondensed light images, wherein the first light receiving element andsecond light receiving element are disposed at positions equidistantlyspaced from a focal point of the condensed light, and generateelectrical signals based on light received by the light receivingelements, wherein each of the first light receiving element and secondlight receiving element includes: a first light receiving area, whichreceives a first portion of the condensed light which includes a opticalaxis of the condensed light; a second light receiving area, whichreceives a second portion of the condensed light which is locatedoutside of the first portion of the condensed light; a third lightreceiving area, which receives a third portion of the condensed lightwhich is located outside of the second portion of the condensed light;and an aberration amount detecting circuit, which detects an aberrationamount by using the output signals of the first light receiving elementand second light receiving element.
 16. The light detecting deviceaccording to claim 15, wherein the aberration amount AB is detected byusing at least one of the following equations:AB=a ₊ −a ⁻AB=(a ₊ +b ⁻)−(b ₊ +a ⁻),AB=(a ₊ +b ⁻ +c ⁻)−(a ⁻ +b ₊ +c ₊),AB=(a ₊ +b ₊ +c ⁻)−(a ⁻ +b ⁻ +c ₊),AB=(a ₊ +b ⁻ +c ₊)−(a ⁻ +b ₊ +c ⁻),AB=(a ₊ +b ₊)−(a ⁻ +b ⁻) where a₊ is an output signal derived from thefirst light receiving area of the first light receiving element, b₊ isan output signal derived from the first light receiving area of thefirst light receiving element, and c₊ is an output signal derived fromthe third light receiving area of the first light receiving element, a⁻is an output signal derived from the first light receiving area of thesecond light receiving element, b⁻ is an output signal derived from thesecond light receiving area of the second light receiving element, andc⁻ is an output signal derived from the third light receiving area ofthe second light receiving element.
 17. A light detecting devicecomprising: a first light receiving element, which receives thecondensed light before the condensed light images; and a second lightreceiving element, which receives the condensed light after thecondensed light images, wherein the first light receiving element andsecond light receiving element are disposed at positions equidistantlyspaced from a focal point of the condensed light, and generateelectrical signals based on light received by the light receivingelements, wherein each of the first light receiving element and secondlight receiving element includes: a first light receiving area, whichreceives a first portion of the condensed light which includes a opticalaxis of the condensed light; a second light receiving area, whichreceives a second portion of the condensed light which is locatedoutside of the first portion of the condensed light; a third lightreceiving area, which receives a third portion of the condensed lightwhich is located outside of the second portion of the condensed light;and a focus correction amount detecting circuit for detecting a focuscorrection amount by using the output signals of said first lightreceiving element and second light receiving element.
 18. The lightdetecting device according to claim 17, wherein said focus correctionamount FO is detected by using any of the following equations:FO=a ₊ +a ⁻FO=(a ₊ +b ⁻)−(b ₊ +a ⁻),FO=(a ₊ +b ⁻ +c ⁻)−(a ⁻ +b ₊ +c ₊),FO=(a ₊ +b ₊ +c ⁻)−(a ⁻ +b ⁻ +c ₊),FO=(a ₊ +b ⁻ +c ₊)−(a ⁻ +b ₊ +c ⁻),FO=(a ₊ +b ₊)−(a ⁻ +b ⁻) where a₊ is an output signal derived from thefirst light receiving area of the first light receiving element, b₊ isan output signal derived from the second light receiving area of thefirst light receiving element, c₊ is an output signal derived from thethird light receiving area of the first light receiving element, a⁻ isan output signal derived from the first light receiving area of thesecond light receiving element, b⁻ is an output signal derived from thesecond light receiving area of the second light receiving element, andc⁻ is an output signal derived from the third light receiving are of thesecond light receiving element.
 19. An optical signal reproducingdevice, which reproduces a signal recorded in an optical recordingmedium, the optical signal reproducing device comprising: a lightdetecting device comprising: a first light receiving element, whichreceives the condensed light before the condensed light images; and asecond light receiving element, which receives the condensed light afterthe condensed light images, wherein the first light receiving elementand second light receiving element are disposed at positionsequidistantly spaced from a focal point of the condensed light, andgenerate electrical signals based on light received by the lightreceiving elements, wherein each of the first light receiving elementand second light receiving element includes: a first light receivingarea, which receives a first portion of the condensed light whichincludes a optical axis of the condensed light; a second light receivingarea, which receives a second portion of the condensed light which islocated outside of the first portion of the condensed light; a thirdlight receiving area, which receives a third portion of the condensedlight which is located outside of the second portion of the condensedlight; and an aberration amount detecting circuit, which detects anaberration amount by using the output signals of the first lightreceiving element and second light receiving element.
 20. An opticalsignal reproducing device, which reproduces a signal recorded in anoptical recording medium, the optical signal reproducing devicecomprising: a first light receiving element, which receives thecondensed light before the condensed light images; and a second lightreceiving element, which receives the condensed light after thecondensed light images, wherein the first light receiving element andsecond light receiving element are disposed at positions equidistantlyspaced from a focal point of the condensed light, and generateelectrical signals based on light received by the light receivingelements, wherein each of the first light receiving element and secondlight receiving element includes: a first light receiving area, whichreceives a first portion of the condensed light which includes a opticalaxis of the condensed light; a second light receiving area, whichreceives a second portion of the condensed light which is locatedoutside of the first portion of the condensed light; a third lightreceiving area, which receives a third portion of the condensed lightwhich is located outside of the second portion of the condensed light;and a focus correction amount detecting circuit for detecting a focuscorrection amount by using the output signals of said first lightreceiving element and second light receiving element.